Fuel cells convert energy that is stored in the molecular bonds of chemical compounds directly into electrical energy. This “cold combustion” differs from the conventional (“hot”) combustion in that the dissociation reactions and reactions of formation on which the energy conversion (redox reaction) is based are carried out spatially separated, and the conversion of the chemical energy into electrical energy is carried out not indirectly by heat generation and conversion of the heat energy into mechanical work.
In the case of a fuel cell, which uses methanol (CH3OH) as a fuel, said methanol being combusted to carbon dioxide and water, the following balance equation can be defined for the conversion of substances proceeding in the fuel cell—independent of the actually proceeding reaction steps:CH3OH+1,5O2->CO2+2H2O(+ΔG).
The useful electrical energy of the fuel cell is given by the Gibbs' enthalpy ΔG of this reaction and amongst others depends on the operating temperature of the fuel cell.
The structure of a fuel cell consists of an anode-sided chamber and a cathode-sided chamber, which are separated from one another by an ion-conducting (liquid or solid) electrolyte. In the anode-sided chamber the (usually hydrogen-containing) fuel is supplied, whereas the reaction substance, usually oxygen, is supplied to the cathode-sided chamber.
Furthermore, reaction products (combustion products), e.g. water and/or carbon dioxide are generated in the fuel cell, which must possibly be discharged partially or fully.
A general distinction is made between acid electrolytes (e.g. sulphuric acid, phosphoric acid) through which positive ions travel from anode to cathode, and alkaline electrolytes (e.g. caustic potash), through which negative ions travel from cathode to anode.
In a fuel cell that uses methanol as a fuel and which has an acid electrolyte, e.g. a proton-conducting membrane, the following anode and cathode reactions can be given:
Anode:CH3OH + H2O->CO2 + 6H+ + 6e−Cathode:1.5O2 + 6H+ + 6e−->3H2O
It can be recognized that H2O in the fuel cell is not only produced as a combustion product and must be discharged on the cathode side, but that H2O together with the actual fuel methanol must also be supplied at the anode side (which cannot be derived from the balance equation).
Since at the cathode more water as is produced as combustion product than must be supplied at the anode, a respective part of this water produced as waste product can at least theoretically be returned to the anode during operation so that the necessity of an external water supply can be avoided.
This water return is, however, connected with several technical problems:                the amount of water that must be supplied at the anode differs from the amount that accumulates at the cathode;        the water accumulating at the cathode usually exists in a different state of aggregation (gaseous) than is needed at the anode (liquid);        the water accumulating at the cathode does usually not exist in pure form but is “polluted” by other combustion products (e.g. carbon dioxide),        the above-mentioned anode and cathode equations are net equations and are a strong simplification of the actual process: in reality, each diffusing proton encloses itself in a hydrate shell and thereby draws a parasitic portion of water from the anode through the electrolyte to the cathode; a more realistic anode equation therefore reads as follows:CH3OH+(1+x)H2O->CO2+6H++6e−+xH2O,wherein the value of x depends on the operating parameters, particularly the temperature.        
It must be emphasized that the parameter x must in no case be neglected (x<<1) but that, on the contrary, water of x>1 and in the scale of 100 . . . 101 comes far more closer to reality than the net balance equation (x=0). In the case of a realistic hydrate shell of 1 to 6 water molecules per proton, x adopts the values 6 to 18.
In order to elucidate which substance quantities must actually be supplied at the anode and cathode, it is useful to define the balance equation in the following (unabridged) form:[CH3OH+(1+x)H2O]+1,5O2->CO2+(3+x)H2O,wherein in order to maintain operation, the substance mixture defined in the square brackets [CH3OH+(1+x)H2O] must permanently be supplied at the anode side.
Similar problems also occur with a fuel cell with alkaline electrolytes. For a DMFC (direct methanol fuel cell) with a hydroxide ion conducting polymer membrane, the following anode and cathode reactions can for instance be defined:
Cathode:1.5 O2 + 3H2O + 6e−->6OH−Anode:CH3OH + 6OH−->CO2 + 5H2O + 6e−
In this case, water does not only develop as a combustion product but it must also be supplied on the cathode side. A return is amongst others aggravated by the fact that the water at the anode side is a water/methanol mixture whose separation is technically very complex. Furthermore, the hydroxide anion, similar to the proton, is enclosed in a hydrate shell so that it is also useful in order to explain the consequences resulting therefrom to state the balance equation in unabridged form:CH3OH+[1,5O2+(3+y)H2O]->CO2+(5+y)H2O.
To maintain operation, the substance mixture [1,5O2+(3+y)H2O] stated in square brackets must proportionately be supplied on the cathode side, wherein the water need (3+y)H2O can in principle be covered by the greater water quantity (5+y)H2O produced at the anode, but the technical realization of the proportional water return is, as mentioned, connected with great technical difficulties.
Due to this general tendency of ions diffused through an ion-conducting electrolyte, described above by means of examples, to surround itself with a hydrate shell and to draw this hydrate shell through the electrolyte (electroosmosis), comparable effects also occur in fuel cells which use a fuel different than methanol.
Thus, a general problem aggravating the structure as well as the operation of fuel cell systems is that substances produced on one side as a reaction product—be this as a result of parasitic transport through the electrolyte (electroosmosis) or as a result of a chemical reaction or a combination thereof—are needed on the other side, wherein a simple return of these reaction products is confronted with various technical problems.